1. Field of the Invention
The present invention relates to the field of acquiring and tracking satellite signals, and more specifically, to the field of acquiring and tracking satellite signals having a low signal-to-noise ratio (SNR).
2. Discussion of the Prior Art
Typically, GPS satellites transmit signal power such that a receiver on the surface of the Earth with clear view of the sky should expect to receive signal power of at least −130 dBm. If ‘u’, is a real, nonnegative, power signal (units of watts), the dBm conversion is equivalent to performing the dB operation after converting the input to milliwatt:y=10*log 10(u)+30  (Eq. 1)
For the transmit signal power details, please, see “GPS Interface Control Document”, ICD-GPS-200, IRN-200B-PR-OOJ, Rev. B-PR, U.S. Air Force, Jul. 1, 1992.
The received signal power may be translated into an SNR across each 50 Baud data bit by taking into account noise sources, receiver losses and bandwidths. The received signal having −130 dBm power corresponds to approximately to a signal having 28 dB SNR across each data bit. At these SNRs levels data the bit error rate (BER) is relatively low, the bit errors are very rare and can be easily detected and corrected via, for example, the data's forward error correction (FEC) capabilities.
In recent years GPS applications have grown into areas where receivers are required to operate successfully i.e. provide position solution, under low SNR conditions. These low SNR situations include, for example, (1) indoor operations; and/or (2) partial signal block conditions. For instance, in a situation where a number of satellites have high and low SNRs, in a situation where satellite signals are partially blocked (like a cell phone capable of receiving partially blocked satellite signals), the received signal power is expected to be substantially reduced. Another important application where the received signal power is expected to be substantially reduced is when one, being indoors, needs to get an accurate time reading from GPS by getting a signal from only one satellite, whereas the position is assumed to be already known. The received signal power target for indoor applications and other partially blocked conditions is better than −150 dBm, that is (20-30) dB lower than the nominal signal power, thus requiring innovative signal acquisition and tracking techniques.
There are a number of operations required of a GPS receiver to successfully use a satellite in a position solution under low SNR conditions. One of these operations is finding the satellite's signal power via a search of the satellite's carrier frequency and code phase bins. This subject area is covered in the U.S. patent application Ser. No. 09/512,243 “Method and Apparatus for Fast Acquisition and Low SNR Tracking in Satellite Positioning System Receivers” by Gary Lennen. The U.S. patent application Ser. No. 09/512,243 “Method and Apparatus for Fast Acquisition and Low SNR Tracking in Satellite Positioning System Receivers” is incorporated herein in its entirety and is referred to hereafter as the patent application #1.
Once the satellite signal power has been found (by applying, for instance, the method of patent application #1), the receiver is required to close carrier and code tracking loops, to align 50 Baud data bit edges, and to attempt a data decode operation. Because of the low SNR conditions, each of these operations should be performed substantially optimally, that is, by optimal usage of the available signal power and without unnecessarily introducing any signal loss. The optimal usage of the available signal power has additional benefits, like optimization of the time and reliability in obtaining a position fix, minimization of the energy used per position fix, and increase in the responsiveness of the GPS receiver.